About miners going out of business is something we can't talk about presently, I don't know if POW will be erased entirely from ethereum blockchain, I've seen coins that has both POW and POS functioning on same network, all we can do is wait and see.
Possibly the ethereum Blockchain team developers may choose to erase the pow and move forward with the pos, this is very much possible as we have seen in others. Basically, the team developers know what they need with this upgrade and will execute accordingly. In my own opinion, they should erase the pow entirely from the ethereum Blockchain because the system seem not effective as much as pos.